The invention is generally related to water distributing systems and devices, and is specifically directed to a center-pivot, field irrigation system capable of operating at low or high water pressure, and a constant-volume sprinkler head for such irrigation systems.
Center-pivot irrigation systems typically comprise an extremely long water conduit "arm", which is pivotally connected at one end to a source of water under pressure. The conduit arm is carried in an elevated position, usually by a plurality of radially spaced wheeled towers which are powered by hydraulic, pneumatic or electrical motors to rotatably sweep the conduit arm through and over a circular field. The conduit arm includes a predetermined number of water sprinkling heads, which are radially spaced over its length and constructed to distribute a spray of water on the circular or annular field area over which they pass.
Center-pivot irrigation systems have strongly and successfully established themselves in the farming community. Although initially expensive, they presently represent one of the most efficient manners of irrigation, insuring that most of the crop receives an adequate supply of water and thus increasing crop yield.
For some period of time, center-pivot irrigation systems have operated at reasonably high water pressure, typically on the order of 70 psi. This has been environmentally and economically unsound, since such levels of operation require more elaborate pumping equipment, as well as conduit and sprinkler heads capable of withstanding such pressures. High pressure equipment is more expensive to operate due to fuel consumption. Further, the extreme pressure causes substantial evaporation of the water for at least two reasons. First, the water is often propelled through the air for significant distances where higher pressures are used, and the more exposure to the air, particularly when it is dry, the greater the degree of evaporation. Secondly, irrigation systems of this type often create a spray by directing a high velocity water jet against a deflector. The resulting spray is a fine mist, at least in part, which is highly subject to evaporation before it reaches the ground, and the problem is severely compounded by windy conditions, which also tend to blow the spray away from the intended area.
Consequently, many of the newer systems have been designed to operate at low water pressure, typically on the order of 20 psi. Lower pressures clearly have the advantage of less operating cost, and there is usually less evaporation under still conditions. However, evaporation and misdirection of the spray pattern have continued to be a problem under windy conditions, resulting in erratic and nonuniform distribution of water over the field. Nonuniform distribution is even more pronounced where differences in elevation occur in the field even where such differences are not great. A severe pressure drop occurs wherever there is any degree of elevational difference in the conduit arm. This results in poor water distribution in the high areas of the field, whereas over watering occurs in the low spots. Thus, the field becomes "spotted" with areas which have received too little or too much irrigation, and much or all of the advantage of low pressure irrigation is lost. This is not, of course, conducive to optimum crop yield.
The inventive irrigation system and sprinkler head therefore are the result of an endeavor to develop a low pressure center-pivot system capable of uniformly distributing water over the field notwithstanding differences in elevation or windy conditions, and that overcomes high percentage water losses due to evaporation.
The irrigation system comprises an elevated conduit arm that is pivotally connected to a stationary point (usually the well pipe), and is powered to rotatably sweep through and over the field. The system further comprises a plurality of sprinkler heads spaced over the length of the conduit arm, each of which is constructed to create a spray formed from water droplets that are large enough to resist being blown off-course by the wind, but not so large as to damage farm plants that may be small and fragile after sprouting and during early development.
Because the area of a circular field increases exponentially as the field radius increases, the system must be properly designed to insure that the sprinkler heads have the capacity to cover the entire field with a sufficient volume of water, and that this predetermined volume is uniformly distributed even without elevation differences or windy conditions. Thus, assuming that the sprinkler heads are equidistantly spaced, each successive head in the radially outward direction generally must have a greater output capacity since the annular area which it overlies is greater than the annular area which next precedes it. Stated otherwise, although the annular band width of all sprinkler head areas may be essentially constant with equidistant spacing, each successive area nevertheless increases appreciably because its effective radius increases. Accordingly, the output capacity of each sprinkler head must be chosen to deliver the proper volume of water per unit of time based on the specific area which it overlies and serves.
Although I prefer increasing the output capacity of successive sprinkler heads as a function of their radial distance from the pivot point, it would be possible to use sprinkler heads of the same output capacity and decrease the spacing therebetween as a function of increasing radial distance from the pivot point. Because the output capacity of my unique sprinkler head can be varied much more easily (due to interchangeability of control components) than can sprinkler head spacing on the conduit arm, the equidistant spacing approach is strongly preferred. This is particularly so since proper water distribution is necessarily conditioned on geographic area, annual rainfall, type of crop and the like. Further, many existing systems already have equidistantly spaced sprinkler heads but can be readily converted to the inventive system.
Having designed the system to be capable of uniform and sufficient water distribution over the entire field, the problem of pressure fluctuations due to differences in elevation can be overcome on an individual sprinkler head basis. This is accomplished through the use of a volume control device within the sprinkler head that maintains a constant volume output even in the face of water pressure fluctuations in the conduit arm. Thus, assuming that water under a predetermined minimum pressure of sufficient volume is always supplied to the conduit arm, the individual sprinkler heads respond to the delivered pressure and distribute the same volume of water in the same spray pattern throughout all phases of the operation.
I have overcome the problem of wind affects by designing a sprinkler head that creates a flow pattern that is less than 360.degree. , and which is always directed into the wind. This is specifically accomplished with a deflector designed to create the desired flow pattern upon impingment by a low pressure water jet. The deflector is mounted for rotation in an essentially horizontal plane and includes a wind-sensitive vane that always keeps the deflector in a position directing the spray into the wind. The deflector is also constructed to be tipped about an essentially horizontal axis, and aileron-like devices are also included which cause the deflector to tip under severe windy conditions, directing the spray pattern somewhat downward as well as into the wind.
I have found that the inventive low pressure, center-pivot irrigation system employing these unique sprinkler heads successfully combats the problem of uneven water distribution and spotty crop production due to differences in elevation, windy conditions, friction loss and water evaporation. Further, since the sprinkler head is designed with component interchangeability in mind, an irrigation system can be custom designed to the conditions of a specific field with very little difficulty.
The inventive irrigation system and sprinkler head include a number of additional advantageous structural features, which will become apparent from the drawings and description below.